The occurrences of asthma continue to rise each year. One in five Americans, roughly 50 million people, suffer from an allergic disease. Asthma and allergies are the most common chronic diseases in this country. There are also many individuals that suffer from chronic bronchitis or emphysema. Many of the individuals afflicted with these ailments use a metered dose inhaler (MDI). The MDI is by far the most common and effective treatment for most respiratory problems.
The most common MDI device currently in use is comprised of a L-shaped dispenser and an aerosol canister. The aerosol canister contains medication and is capable of delivering a finite number of metered doses of pressurized medication. The aerosol canister is inserted into the L-shaped dispenser with the hollow stem of the canister pointing downward.
The patient self administers the medication. The open end at the bottom of the L-shaped dispenser is placed near the patient's mouth before the medication is released. A single push on the canister releases a single dose of the medication in the form of an inhaled aerosol. The downward pressure is placed on the top of the aerosol canister which protrudes from the opening in the L-shaped dispenser in which the canister was inserted. The downward pressure forces the hollow tube of the canister back into the canister which releases a single dose of the medication.
A significant deficiency of MDI's is the inability of the patient to precisely determine how many metered doses of medication remain in the aerosol canister. A patient without medication that has a serious respiratory problem can find themselves in a life threatening situation.
The consequences of an untreated asthma attack can be deadly. A serious asthma attack can result in a combination of swelling of the lungs and muscular constriction of the airways with little or no air movement beyond the obstruction. The lungs become over inflated since the patient cannot exhale air within their lungs. The air remains trapped in the lungs and the routine exchange of oxygen with carbon dioxide cannot occur. Carbon dioxide builds in the bloodstream. The situation is life threatening.
Each year in the United States an estimated four thousand deaths occur due to asthma alone. A key factor putting the patient at risk is failure or inability to comply with the medication program suggested by the doctor.
There is currently no simple procedure to determine the number of metered doses of medication remaining in the aerosol canister. The recommended procedure to check for an empty or nearly empty aerosol medicine canister is to remove the canister from the L-shaped delivery device to see if the canister floats or sinks in a glass of water. This method may be difficult to execute when the patient is away from home. Most patients simply shake the aerosol canister from the L-shaped dispenser in an attempt to determine if it is empty or nearly empty. This method is highly inaccurate and can result in the patient believing that they are continuing to receive full doses of medication well after the canister has passed the point of effective delivery. This is true because when depressed, the pressurized canister will continue to expel some amount of medication well after the point where it delivers the full metered dose. On the other hand, the patient might also believe that there is a good deal of medication remaining in the canister and fail to take additional medication along when leaving home.
The situation is further complicated when less instantaneous, longer acting, preventative medications are considered. A patient using a quick acting asthmatic medication, such as bronchodialator like albuterol (Proventil or Ventolin) can normally determine quite quickly if they are getting a proper dose of medication, since the patient expects to get relief from the asthma attack almost immediately. Slower acting drugs, such as inhaled steroids or cromolyn sodium (Intal) target the inflammation in the lungs, not the constricted muscles of bronchial tubes. Most preventative medications require several doses over a period of days to be fully effective. It is virtually impossible for a patient to determine if they are receiving the proper amount of a preventative medication at the time of inhalation by the immediate effect on their body.
Electronic, mechanical, and electro-mechanical counting devices have found their way into many aspects of modern society. Mechanical counting devices are comprised of a series of moving parts that can calculate information. These devices can be as simple as a mechanical hand counter used by an usher at a gate to track the number of patrons entering an event. Mechanical counting devices are often found with analog mechanical displays. These displays can be use to indicate information such as a the current time or a value. One form of an analog mechanical display is one or more moving hands that points to a value that is listed on a background display. It is also common to see analog mechanical displays in the form of synchronized rotating wheels with decimal numbers displayed on the external rim of each wheel in which only one decimal number per wheel is visible at any one time. Internal gear or other moving parts are used to synchronize and increment the hand or wheels of the display.
It is common to find a mechanical reset button associated with a mechanical counter that when pushed will align the internal mechanical parts to point to a predetermined starting value to reset the counter. It is also common on a wrist watch to find one or more knobs that can be turned to rapidly change the display to some desired value.
The relatively large number of moving parts typically make mechanical counting and computing devices more difficult to manufacture; especially when a miniature device is required. One clear advantage of mechanical calculators is the ability to construct them without the need of a power supply, such as a battery.
VLSI (Very Large Scale Integration) and low power CMOS (Complementary Metal Oxide Semiconductors) have made it possible to create extremely small low power electronic and electro-mechanical computing devices with a wide range of functions and capabilities. Electronic and electro-mechanical devices can take the form of a wristwatch or a camera. Wristwatches can be found that are totally electronic with digital readout such as LED (Light Emitting Diode) displays or LCD (Liquid Crystal Displays). Wristwatches can also be found that are a combination of electronic counting circuits and mechanical analog displays in the form of an hour and minute hand. One clear disadvantage of the electronic or partially electronic watch is the need to periodically replace the battery. Fortunately, battery replacement may not be necessary for years which makes it an acceptable burden and perhaps easier than winding a wristwatch every few days.
Small, more complex electronic devices can also be found today. They may take the form of a computer with not just counting circuits but a complex set of programmable instructions and a time-of-day clock. When connected to peripheral input/output devices, such devices can provide a wealth of functions and handle very complex tasks. One place we see this is in small compact cameras and video recording equipment.
Despite a limited life, due to the finite power provided by a battery, electronic and electro-mechanical counting devices are normally much easier and less complicated to manufacture than mechanical devices. This is certainly true when one compares function to the number of moving parts and size. Electronic and electro-mechanical devices can normally be produced in very small compact packages. Again, this is primarily due to the advances in VLSI circuit design technic and that they can be produced with few, if any moving parts.
An electronic device offers the ability to add additional functions at low cost. Unlike a mechanical counter, a power supply normally in the form of a battery is required. The use of low power CMOS, a LCD display, and a small cylindrical battery can produce a device that can last a very long time without the need to replace the battery. This period of time can be extended even further if a timed auto-shutdown circuit is employed, which allows the display and any non-critical circuits, other than the latches holding the count to be powered off, in order to prolong the life of the battery. Inexpensive, non-replaceable batteries can be found in some applications, especially for disposable devices. This can further reduce cost and complexity in manufacturing.
The counting device can be made as simple or as elaborate as desired. Naturally, there is a tradeoff between function and cost. Electronic computers and counters have been equipped with a circuit that drives a small plastic speaker. We find such speakers on wristwatches where they produce an audible alarm. We also find automatic power off circuits that cut power to the display after some timed interval to conserve power.